Project Summary This project aims to characterize at the molecular, cellular and organismal levels the pathophysiology of clonal histiocytic disorders, exemplified by Langerhans Cell Histiocytosis (LCH) and Erdheim-Chester Disease (ECD). These diseases display a considerable heterogeneity in terms of prognostic and clinical presentation, and are characterized by the occurrence of neurodegenerative syndromes and liver and lung fibrosis, the mechanisms of which are mysterious. Their pathophysiology is largely unknown. Two recent discoveries offer an opportunity to re-evaluate the pathophysiology and molecular underpinning of clonal histiocytic disorders, improve their diagnostic, prognostic, and offer new therapeutic strategies. First, the discovery of somatic mutations of the RAS-ERK pathway in >70% of cases of LCH and ECD by us and others has led to new therapeutic approaches based on the use of BRAF inhibitors and has renewed interests in efforts toward solving the pathophysiology of these diseases. Second, an important revision of our understanding of myeloid development stems from recent works from our laboratory and others which have revealed that the tissue- resident macrophages found in adult mice originate from early hematopoietic progenitors from the yolk sac independent of hematopoietic stem cells (HSCs) and persist within their tissue of residence in adults. Following on this work, we made the novel hypothesis that somatic BRAF mutations in yolk sac progenitors may causes histiocytoses and we have developed in vivo models that allow to test this hypothesis experimentally. Our preliminary results indicate that conditional expression of a BRAFV600E allele in yolk sac hematopoietic progenitors in vivo does results in the accumulation of BRAFV600E macrophage clones in various tissues, and is responsible in particular for a neurodegenerative syndrome in adult mice, that recapitulates for the first time one of the most intriguing and adverse phenotypic characteristics of the human histiocytic neoplasms. We will (Aim 1) will investigate the molecular and cellular mechanisms that underlie the development of brain neurodegenerative disease and identify molecular pathways that drive pathological histiocytes, (Aim 2) determine the pathological roles of BRAFV600E macrophages outside the brain in vivo and in particular in liver fibrosis, and (Aim 3) investigate the efficiency of BRAF inhibitor administration in the murine models. These experiments will provide a proof of concept that a mutations in yolk sac progenitors can cause an histiocytic disease in adults, help elucidate the pathophysiology and mechanism of histiocytoses and some of their most adverse complication such as the neurodegenerative syndrome, and reveal novel therapeutic targets to be used in conjunction with, or alternatively to, BRAF inhibitors.